On leaving the breakdown stand and prior to entering the finishing stand, the slab pauses approximately 1 to 11/2 minute on a waiting table. Its dimensions at this stage are commonly 30 to 50 mm thick by 1 to 2 m wide by up to 70 m long.
In addition to a considerable overall cooling (of the order of 100.degree. C.), there occurs a more pronounced cooling of the slab verges due to their greater heat exchange surface area (in effect).
Experience shows that the zone affected by this localized cooling can extend more than 70 mm inwards of an edge and that the mean temperature differential through the thickness can be as high as 75.degree. C. over this distance, the mean temperature remaining roughly constant beyond the 70 mm verge, as far as 70 mm from the opposite edge of the slab.
Such localized cooling involves three major disadvantages, as follows:
maintaining a minimum temperature throughout the strip at the finishing stand (rolling mill output temperature) requires superheating the slab in the reheating furnace;
a nonuniform thermal profile at the end of rolling leads to nonuniform metallurgical properties over the width of the strip;
and the cold verges bring about greater wearing of the rolls of the finishing stand (tapering)--a phenomenon entailing production engineering constraints on strip production. This is one factor limiting the greater development of hot charging.
Similar problems arise in heavy plate rolling.
The verge temperature gradient could be reduced by installing a tunnel over the waiting table, so as to slow the cooling of the slab as a whole. Moreover, the thermal gradient at the verges could be more substantially reduced by adding burners to preferentially heat the edges. However, such flame heating does not provide the heat output profile needed to solve the problem at hand.
One method proposed in the prior art to cancel the verge differentials is to induction reheat portions of the products using inductors with a U-shaped magnetic core, placed above and below the slab verges: the product advances through the air gap formed between two superposed inductors whose magnetic poles of opposite polarity face one another.
There are also prior art means for related applications consisting of inductors with C-shaped magnetic cores through the opening whereof the product to be heated (strip verges, wires or bars) is made to pass, the ends of the core facing each other and advantageously serving as carriers for the windings of the electrical excitation current conductor, such as to constitute opposite-polarity, wire wound magnetic poles: FR-A-No. 2 489 645 (EDF), FR-A-No. 2 555 353 (CEM) and EP-A-No. 0 170 556 (EDF).
The object of the invention is to provide a new induction "reheating" system having a greater efficiency than the prior art device.